Mobile radio communications systems, for example cellular telephony or private mobile radio communications systems, typically provide for radio telecommunication links to be arranged between a plurality of subscriber units, often referred to in the art as mobile stations (MSs). The term mobile station (MS) generally includes both hand-portable and vehicular mounted radio communication units, radiotelephones and the like.
Mobile radio communications systems are distinguished from fixed communications systems, such as the public switched telephone networks (PSTN), principally in that mobile stations can move in geographical location to accompany their user and in doing so encounter varying radio propagation environments.
Mobile radio communications systems and mobile stations used in them may operate in one of two main modes, namely a trunked mode of operation (TMO) and a direct mode of operation (DMO). TMO communications use the infrastructure supplied by the system operator, especially base transceiver stations (BTSs), to deliver communications to and from the MS of each user or subscriber serviced by the system. Resources available in TMO are shared between the many MSs using the system. Systems operating in TMO are often referred to as cellular because a multiplicity of BTSs provide service to MSs in a network of overlapping regions known as cells. In contrast, DMO is a method that provides the capability of direct communication between two or more MSs without use of any associated operator's infrastructure. Some MSs may be dual mode operating using either TMO or DMO.
Methods are known for the location of a MS operating in a TMO cellular communication system to be determined. Such methods include a so-called TDOA or time difference of arrival method and a GPS (global positioning system) based method. The TDOA method involves sending signals from a given MS to at least three BTSs the locations of which are known precisely or receiving in a MS signals from at least three such BTSs. In the case where the signal from the MS is received by the three BTSs, using accurate time reference (GPS usually), and measurement of the transit delay for the signal to reach each BTS of the signal from the specific MS, the system can calculate the location of the MS.
In emergency situations, it should be possible to search for lost people who are accompanied by an active MS by estimating the location of the MS. Examples of situations where such searches are likely to be necessary include those needed where people become lost at sea or in a forest, or on mountain, or where they have become partially or totally buried following an explosion, avalanche etc. Unfortunately, the typical accuracy of the known location estimation methods which have been implemented in practice is only about several hundred metres and such method rely on the existence of a BTS infrastructure in the area where the search is to be made. In consequence, these known methods are of limited help where location precision much better than 100 metres is needed.